This application generally relates to interferometry, and in particular, to an all-reflective, radially shearing interferometer.
Interferometry is a technique for diagnosing the properties of two or more waves by studying the pattern of interference created by their superposition. There are various types of known interferometers. These may include, for instance, Newton, Fizeau, Twyman-Green, lateral shearing, and radially shearing interferometers.
One known radially shearing interferometer beam splits an entering wavefront into two beam paths which have different magnification. The two beams are then recombined and interfered. See, e.g., Daniel Malacara, Optical Shop Testing, John Wiley & Sons, Inc., 1992, Chapter 5, herein incorporated by reference.
FIG. 1 illustrates a schematic of conventional radially shearing interferometer 100 which includes refractive elements. As illustrated, interferometer 100 is configured in a cyclical geometry. For clarity, light rays have been traced in only one direction.
Collimated light beam 105 from distant object O enters entrance pupil 107 of interferometer 100 and impinges upon beam splitter 110. A portion of beam 105 passes through beam splitter 110 and a portion of beam 105 is reflected by beam splitter 110, yielding first beam portion 115 and second beam portion 125, respectively. Interferometer 100 is known as a “common path” design since both first and second beam portions 115, 125 traverse the same path but in reverse order, as described in the following paragraphs. In a common path configuration, the optical path lengths of the two paths are identical. This feature becomes more important as the spectral width of the light increases and the corresponding coherence length decreases.
First beam portion 115 passes through beam splitter 110. Next, first beam portion 115 passes through first lens 120, is reflected by first mirror 130 through intermediate image location at 135, is reflected by second mirror 140, is re-collimated by second lens 150 and then passes back through beam splitter 110 to exit pupil location 160. First and second mirrors 130, 140 are both flat mirrors. First beam portion 115 experiences a spatial magnification, M, given by the ratio of the focal length of second lens 150 divided by the focal length of first lens 120.
Second beam portion 125 is reflected from beam splitter 110. Next, second beam portion 125 passes through second lens 150, is reflected by second mirror 140 through intermediate image location at 135, is reflected by first mirror 130, is re-collimated by first lens 120 and is reflected by beam splitter 110 to exit pupil 160.
Second beam portion 125 experiences a spatial magnification,
      1    M    ,given by the ratio of the focal length of lens 120 divided by the focal length of second lens 150. A sensor or other detector (not shown) may be located at exit pupil 160 to record the interference pattern generated by the first and second beam portions 115, 125.
First and second lenses 120, 150 scale beams 115 and 125 with reciprocal magnifications depending on the direction of the light through the lenses. This refractive interferometer design, however, is limited in spectral range due to the dispersion of the glass material of the lenses. In addition, wavefront correction is complicated by the lenses. As such, to correct aberrations of the refractive elements adequately, the focal ratio (or f-number) of the lenses must be large (e.g., greater than about F/8), multiple lenses are needed, and/or aspherical lenses are utilized.
Thus, an improved radially shearing interferometer is desired which overcomes the aforementioned drawbacks.